Recording materials using an electron-donating colorless dye and an electron-accepting compound have already found popular recognition as pressure-sensitive paper, heat-sensitive paper, light- and pressure-sensitive paper, electro thermo-recording paper, etc. For example, they are disclosed in detail in British Pat. No. 2,140,449, U.S. Pat. Nos. 4,480,052 and 4,436,920, Japanese Patent Publication No. 23922/85, and Japanese Patent Application (OPI) Nos. 179836/82, 123556/85, and 123557/85 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
The recording materials must have properties of (1) producing a sufficiently high color density of the developed image at a sufficiently high speed, (2) forming no fog, (3) producing a developed imager which retains sufficient fastness after color development, (4) producing a developed image of an appropriate hue, and showing an aptitude for copying machines, (5) having a high signal to noise (S/N) ratio, (6) producing a developed color image having a sufficiently high chemical resistance, and so on. However, recording materials which satisfy all of these requirements to perfection have not been obtained yet.
In particular, studies on improvements in characteristics of such recording materials have been actively pursued in response to the speeding-up of the recording system and the diversification of requirements in recent years.
The present inventors have studied electron-donating colorless dyes and electron-accepting compounds with respect to such factors as oil solubility, degree of solubility in water, distribution coefficient, pKa, polarity of substituents, position of substituents, polymerizability, and alterations of crystallinity and solubility with a view to developing highly desirable recording materials and raw materials used therefor.
During the course of this study, they have studied the polymerization of a colorless dye, in order to develop of a polymerized colorless dye. Their invention relating to polymerization of a colorless dye having an amide group, which has high crystallinity, such as 2-acrylamide-6-diethylamino fluoran, etc. is the subject of copending U.S. patent application Ser. No. 06/872,785. As a result of further studies, it has now been found that a specific polymerized colorless dye, the dye of the present invention, manifests outstanding qualities.
Further, the method according to this invention exhibits a great advantage in the production of a dispersion for use in a recording material.